1. Field of the Invention
The present invention generally relates to a rocker arm adapted to be rockingly driven by a cam for selectively opening and closing a valve mounted on a cylinder head of an internal combustion engine and, more particularly, to the mounting of an adjustment screw for adjusting the opening of the valve.
2. Description of the Prior Art
The rocker arm currently in use is generally available in two models depending on the position of the pivot fulcrum about which the rocker arm undergoes a rocking motion; a screw-locked model in which the adjustment screw is fixed on the arm body and a valve drive model in which the adjustment screw is held in abutment with a post on the engine cylinder head. In addition, each of those models is also available in two types; a center pivot type, in which the rocker arm is rockingly supported at a generally intermediate portion thereof, and an end pivot type in which the rocker arm is pivotally supported at one of its opposite ends through a pivot element secured to such one of the opposite ends.
In the screw-locked model, the rocker arm body is formed with a screw hole in which the adjustment screw is threaded and is then locked in position by means of a fastening nut. This adjustment screw employed in the end pivot type has one end formed with a pivot piece. On the other hand, the adjustment screw employed in the center pivot type has one end formed with a valve abutment. In either type, the adjustment screw is utilized to adjust the opening of a valve body on the engine cylinder head. Accordingly, if the adjustment screw once locked in position is undesirably loosened, not only does the controlled opening of the valve body vary correspondingly, but also it will constitute a cause of generation of noises and vibrations.
Hitherto, the adjustment screw employed in the rocker arm is locked in position on the rocker arm body by means of a single fastening nut with a fastening torque controlled carefully. However, in order for the screw fastening structure to function assuredly, it is necessary for a pre-tensioning force for urging in an axial direction of the screw to be given and retained.
It has, however, been found that with the single fastening nut, the adjustment screw cannot be fastened sufficiently and, therefore, the pre-tensioning force that ought to be retained by, for example, the effect of friction decreases for any reason, accompanied by an incipiency of the adjustment screw to undo. The loosening of the adjustment screw is due to various causes. So far as the single fastening nut is employed as discussed above, for example, depression of the nut seat, on which the single fastening nut threadingly mounted on the adjustment screw is firmly fastened, tends to proceed and, in the worst case, the adjustment screw will be eventually loosened and/or the backlash will undesirably occur between male threads and female threads.
In recent years, the rocker arm of a type prepared from a plate metal by the use of a press work to represent a generally inverted U-sectioned configuration is increasingly employed since as compared with the rocker arm prepared by the use of a metal casting technique, it has numerous advantages such as a light-weight feature, a reduced number of manufacturing steps, a reduced cost of manufacture and others. The adjustment screw mounting structure discussed hereinabove is employed even in this rocker arm. However, the rocker arm of the type prepared from the plate metal has a major problem: Specifically, in the rocker arm of the type prepared from the plate metal, the depth of the screw hole for receiving the adjustment screw is more or less limited due to the limited wall thickness available and, accordingly, it is very difficult for the adjustment screw to be locked in position without accompanying a loosening.